Multilayer films have been utilized for the packaging of "oxygen-sensitive products", such as lettuce, i.e., products which exhibit lower shelf life in the presence of either too much oxygen in the package, or too little oxygen in the package. In such multilayer films used for packaging oxygen-sensitive products, the oxygen transmission rate, and even the carbon dioxide transmission rate, are of primary importance, especially in the packaging of such oxygen-sensitive products as vegetables, fruits, and cheese. For example, in the packaging of precut lettuce, the presence of too much oxygen in the package results in enzymatic browning of cut surfaces, known as pink ribbing. On the other hand, if the concentration of oxygen in the package is too low, the lettuce tends to spoil due to anaerobiosis.
Cut lettuce is frequently packaged using vertical form fill and seal (VFFS) equipment. Produce packaging films used on VFFS equipment have been composed from a variety of materials, including polyethylene, ethylene vinyl acetate copolymer, ethylene methyl acrylate copolymer, blends of polyethylene and ethylene vinyl acetate, and polypropylene. Such films are typically multilayer films.
Furthermore, different products, such as lettuce and broccoli, exhibit maximum shelf life when packaged in films having oxygen transmission rates substantially different from one another. There are many oxygen-sensitive products having a shelf life which is dependent upon the oxygen transmission rate of the package within which they are contained, and the optimum oxygen transmission rates for the packaging of these products varies widely. As a result, it is desirable to provide a process for producing a variety of multilayer films having differing oxygen transmission rates.
Gas transmission rates for vegetable packaging films have traditionally been tailored to a desired level by varying the overall thickness of the multilayer film. That is, in order to achieve a desired, relatively high oxygen transmission rate, a thinner film is produced. Such downgauging is often done at the expense of film strength and abuse resistance. Conversely, film structures which are abuse-resistant and machinable generally lack the desired level of gas permeability and sealing properties required for such applications as use in vertical form fill and seal equipment. It is therefore desirable to provide a film which combines abuse resistance with a relatively high oxygen transmission rate.
Moreover, it would be desirable to utilize a film structure and composition which permits structural and compositional changes so that a set of multilayer films of a substantially singular thickness exhibit a relatively wide spectrum of oxygen transmission rates and carbon dioxide transmission rates. In this manner, overall film physical properties can be kept substantially constant, while at the same time films can be tailored to the optimal oxygen transmission rates and the optimal carbon dioxide transmission rates of a variety of oxygen-sensitive products.
Furthermore, it would be advantageous to provide such a film with a composition and structure which has desirable sealing characteristics, i.e., does not exhibit burn through, does not pucker at the seal, exhibits good hot tack, and seals quickly and at relatively low temperature, while simultaneously permitting optimization of oxygen and carbon dioxide transmission rates, via the ability to vary the structure and composition of the multilayer film, without substantially increasing or substantially decreasing the overall thickness of the multilayer film.
It is also important for multilayer films used in vertical form fill and seal equipment to have surface layers sealable with hot bar and impulse type sealing systems. Other desirable attributes are abuse resistance, clarity, and a modulus sufficient that the film has good machinability on vertical form fill and seal equipment.
Prior art films useful for the packaging of oxygen-sensitive products have been found to lack the combination of desirable OTR for maximum shelf life, machinability properties to run on vertical form fill and seal equipment, high hot tack strength, as well as optical properties such as high gloss and low haze.
Films used on vertical form fill and seal (VFFS) equipment are subjected to at least two different kinds of seals being produced during the packaging of the product, i.e., a longitudinal or vertical seal, and horizontal bottom and top seals. Accordingly, in the construction of such packages, characteristics such as the temperature required to-form the seal, adequate heat resistance to prevent burn through, and a very high percentage of continuous (nondefective) seals, are all important in assessing the performance of any given film for the packaging of oxygen-sensitive products.